Conventionally, a method in which a cDNA library or the like is analyzed has been known as one of analysis methods of genes. The cDNA library is prepared by purifying an mRNA from a cell, and synthesizing a cDNA from this mRNA. In this case, a very small amount of the mRNA can usually be purified from the cell. In a case where the mRNA is purified from cells that exists in mass amount in a living body, it is possible to obtain a sufficient amount of the mRNA for synthesizing a cDNA library by using a large amount of cells for the purification of the mRNA. However, if the mRNA is to be purified from a cell that is in a very small number in the living body (for example, a stem cell, germ cell or the like), there is a great limit in the amount of the mRNA that can be used for synthesizing the cDNA library. In this case, there is a need to amplify the purified mRNA. Accordingly, methods for amplifying a trace amount of mRNA have been conventionally developed.
As the mRNA amplification method as above, a method which amplifies the mRNA by PCR is generally adopted. The following describes more details of the mRNA amplification method. First, mRNA is purified from a cell. Next, cDNA is prepared by reverse-transcription. Subsequently, a double strand DNA is prepared by using a DNA polymerase and the cDNA as a template. This double strand DNA is then amplified by PCR. Finally, the amplified double strand. DNA is treated with an RNA polymerase so as to prepare the mRNA. In this way, an amplified mRNA is obtained (for example, see Patent Document 1).